1. Field of the Invention
The present invention relates to a capacitance difference detecting circuit that detects a change in capacitances of two capacitors and a MEMS sensor that detects an applied angular speed or acceleration according to the change in the capacitances of the two capacitors.
2. Description of the Related Art
In recent years, MEMS sensors (an angular speed sensor and an acceleration sensor) formed by MEMS (Micro Electro Mechanical Systems) are in practical use. The MEMS sensors of this type have a substrate, a mass body disposed displaceably relative to the substrate and oscillating in a vertical direction of the substrate, and two capacitors formed between the substrate and the mass body. They detect, as a change in capacitances (difference between capacitances) of the two capacitors, the displacement of the mass body relative to the substrate caused by a coriolis force (corresponding to an applied angular speed or acceleration) applied in a direction perpendicular to an oscillation direction.
A MEMS sensor of this type detects the change in capacitance caused by the coriolis force as, for example, a difference in delay time that changes according to the capacitance (charge/discharge to the capacitors). Specifically, two nodes to which the two capacitors are connected are supplied with pulse signals respectively. The delay times of the pulse signals differ depending on the capacitance difference. Next, a pulse signal with a pulse width corresponding to the difference between the delay times thereof is generated, and a voltage of this pulse signal is smoothed. Then, the MEMS sensor detects the change in DC voltage resulting from the smoothing as an angular speed or acceleration.
Further, Japanese Unexamined Patent Application Publication No. Hei 8-278336, for example, has disclosed technique for forming a resonance circuit with use of a capacitor and detecting as the change in capacitance a phase difference between an oscillating signal outputted from the resonance circuit and a reference oscillating signal outputted from a reference oscillator.
Each element of the MEMS sensor can be made very small because it is formed by the use of semiconductor manufacturing technology. However, the capacitance of a capacitor as a constituent of the MEMS sensor is also small, so that an amount of change in the capacitance caused by the coriolis force is small. This results in lowered detection sensitivity. With an extremely small amount of change in capacitance, only a small change is caused in delay time, and thus the DC voltage hardly changes. This may cause a failure in detection of a small angular speed and acceleration. Specifically, the capacitance of a capacitor used in the MEMS sensor is as small as several pF, and the change in capacitance of one capacitor caused by the coriolis force is several tens fF. Accordingly, a difference between capacitances of two capacitors that change according to the angular speed and acceleration is several aF to several tens aF. In other words, a highly sensitive MEMS sensor which can detect the capacitance difference of several aF is required.
Moreover, in the aforementioned technique disclosed in Japanese Unexamined Patent Application Publication No. Hei 8-278336, there is only one resonance circuit whose capacitance changes by an external force. Therefore, it is not possible to form a capacitance difference detecting circuit that detects a difference between capacitances of two capacitors that change by an external force. Nor is it possible to form a MEMS sensor having two capacitors between a substrate and a mass body to detect an angular speed or acceleration according to capacitances each changing according to the coriolis force.